This invention relates to control of sticking in high speed twin roll casting, particularly in the casting of aluminum alloys.
In twin roll casting, hot metal is fed through a feeding nose into a convergent cavity formed between the walls of two rotating rolls. Heat is extracted from the melt into the roll shells which are water cooled from the inside. Solidification is completed within a distance between the nose tip and the roll nip (the casting zone) and the solid metal is rolled over an arc of a given length (the rolling zone). The length of the rolling zone depends on the type of caster and operating conditions. In the rolling zone, the strip is reduced up to 50% depending on the length of the rolling zone and the roll diameter.
High speed twin roll casting offers a high productivity and the potential of producing high performance sheet products for the future aluminum market. In this process, the length of the rolling zone is relatively short. A problem associated with this process, which occurs in the absence of any applied parting layer, is strip sticking to the rolls for most commercial aluminum alloys. When sticking occurs, the strip adheres to either or both of the rolls somewhere along the arc of contact. The only aluminum alloys that can be cast without the sticking problem are Al--Mg alloys that have Mg concentrations of about 2% by weight or higher.
A usual cure for the sticking condition is to apply a parting layer, such as graphite powder, to the roll surface. However, such parting layer application requires a delicate control of the process, and reduces the high solidification rate, which is one of the inherent advantages of the process.
U.K. Patent Application GB 2,119,294, published Nov. 16, 1983 describes a method of making sheets of aluminum suitable for ironing in which "galling" problems during processing are solved by treatment in an oxidizing atmosphere. In this process, the oxidizing atmosphere is used in the heat treatment applied to the solidified product of the rolling operation.
It is the object of the present invention to provide a simple method of preventing sticking while maintaining the high speed of a high speed twin roll casting operation.